Hearing devices comprise electronic components that must be supplied with electric energy during operation. This is achieved by either providing a regular battery or by a rechargeable battery within the housing of the hearing device.
For rechargeable batteries, it is well known that the initially available capacity diminishes after repeated full discharge/charge cycles. Other circumstances may also affect available capacity of a rechargeable battery, as for example when rechargeable batteries are stored over long time at a fully charged state (as when being left in the charger after a charging cycle has been completed), or when rechargeable batteries are discharged below a safe low voltage level or are charged above a maximum charging level. Rechargeable batteries reaching about 80% of its initial or nominal capacity are deemed to be defect and need to get replaced.
The reason for the gradual deterioration of a rechargeable battery is due to irreversible physical and chemical changes which take place with usage and with age.
From time to time, all hearing devices are checked for a flawless operation by a hearing device professional. On this occasion, a defective rechargeable battery—or one that is about to reach its end of life—should also be replaced. While a defective rechargeable battery might be easily identified, a rechargeable battery that is about to reach its end of life is not easily identifiable. Therefore, every rechargeable battery must undergo specific tests in order to obtain assurance with regard to the question whether the rechargeable battery may still remain in the hearing device or whether it must be replaced because a battery failure is likely to occur before the next visit at the hearing device professional's office. Such tests incorporate additional measurements and are therefore time consuming.
WO 2004/084582 A1 describes a rechargeable battery for a hearing device comprising a memory to store charging control parameters for an optimized charging algorithm in order to positively influence total battery life time.
US 2012/0130660 A1 discloses a power consumption monitor in a hearing device that is used to monitor the charge of a battery to predict and indicate to the user when it is time to charge the battery again to allow uninterrupted operation of the hearing device.
Although the known techniques allow to optimally operate a hearing device in terms of energy consumption and energy availability, respectively, the known techniques do not provide any indication as to when a rechargeable battery being used to provide energy to the hearing device must be replaced because the rechargeable battery has reached its end of life.
It is therefore an object of the present invention to provide a method of monitoring state of health of a battery.